The United States declassified a program to build nuclear-powered spaceships to send astronauts to Mars and may work on the project with Russian scientists.

"This giant leap in technology can be equated to the progress made when man went from riding horses to driving automobiles," Sen. Pete Domenici, R-N.M., said Monday. He is chairman of the Ninth Symposium on Space Nuclear Power Systems being held here this week.Russian scientists were expected to unveil a mockup nuclear rocket engine at the symposium Tuesday. They are advocating a joint venture with the United States to produce a nuclear engine that could support a manned flight to Mars.

The Russians have given their U.S. counterparts previews of the system, said Nikolai Ponomarev-Stepnoi, deputy director of the I.V. Kurchatov Institute of Atomic Energy in Moscow.

The U.S. program will be managed by the Air Force's Phillips Laboratory in Albuquerque. The laboratory's Lt. Col. Roger Lenard said portions of the project were declassified Monday because it's hard to implement "breakthrough technology in a classified world."

But Steve Aftergood of the Federation of American Scientists complained that it has been going on secretly since 1987. "As a result, even the best-intentioned critics or supporters can speak only in generalities," he said.

Current rocket engines use chemical propulsion, burning a fuel such as liquid hydrogen and an oxidizer to provide thrust, said Gary Bennett, deputy director of the Transportation and Platforms Division at the National Aeronautics and Space Administration.

In a nuclear thermal propulsion engine, hydrogen is injected into an atomic reactor that heats it. The hydrogen is then expelled out a nozzle at a high velocity.

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